# Gauss's constant

In mathematics, Gauss's constant, denoted by G, is defined as the reciprocal of the arithmetic–geometric mean of 1 and the square root of 2:

${\displaystyle G={\frac {1}{\operatorname {agm} \left(1,{\sqrt {2}}\right)}}=0.8346268\dots .}$

The constant is named after Carl Friedrich Gauss, who on May 30, 1799 discovered that

${\displaystyle G={\frac {2}{\pi }}\int _{0}^{1}{\frac {dx}{\sqrt {1-x^{4}}}}}$

so that

${\displaystyle G={\frac {1}{2\pi }}\mathrm {B} \left({\tfrac {1}{4}},{\tfrac {1}{2}}\right)}$

where Β denotes the beta function.

Gauss's constant should not be confused with the Gaussian gravitational constant.

## Relations to other constants

Gauss's constant may be used to express the gamma function at argument 1/4:

${\displaystyle \Gamma \left({\tfrac {1}{4}}\right)={\sqrt {2G{\sqrt {2\pi ^{3}}}}}}$

Alternatively,

${\displaystyle G={\frac {\left[\Gamma \left({\tfrac {1}{4}}\right)\right]^{2}}{2{\sqrt {2\pi ^{3}}}}}}$

and since π and Γ(1/4) are algebraically independent with the Gauss's constant, Gauss's constant is transcendental.

### Lemniscate constants

Gauss's constant may be used in the definition of the lemniscate constants, the first of which is:

${\displaystyle L_{1}\;=\;\pi G}$

and the second constant:

${\displaystyle L_{2}\,\,=\,\,{\frac {1}{2G}}}$

which arise in finding the arc length of a lemniscate.

## Other formulas

A formula for G in terms of Jacobi theta functions is given by

${\displaystyle G=\vartheta _{01}^{2}\left(e^{-\pi }\right)}$

as well as the rapidly converging series

${\displaystyle G={\sqrt[{4}]{32}}e^{-{\frac {\pi }{3}}}\left(\sum _{n=-\infty }^{\infty }(-1)^{n}e^{-2n\pi (3n+1)}\right)^{2}.}$

The constant is also given by the infinite product

${\displaystyle G=\prod _{m=1}^{\infty }\tanh ^{2}\left({\frac {\pi m}{2}}\right).}$

It appears in the evaluation of the integrals

${\displaystyle {\frac {1}{G}}=\int _{0}^{\frac {\pi }{2}}{\sqrt {\sin(x)}}\,dx=\int _{0}^{\frac {\pi }{2}}{\sqrt {\cos(x)}}\,dx}$
${\displaystyle G=\int _{0}^{\infty }{\frac {dx}{\sqrt {\cosh(\pi x)}}}}$

Gauss' constant as a continued fraction is [0, 1, 5, 21, 3, 4, 14, ...]. (sequence A053002 in the OEIS)

## Record progression

Several world record attempts have been made to calculate the most digits of Gauss' constant or one of the lemniscate constants. Usually, the arclength of a lemniscate of radius = 1, or twice the first lemniscate constant, is calculated. Here is a chart for twice the first Lemniscate constant.[1]

Date Name Number of digits
April 3, 2016 Ron Watkins 200 billion
Feb 9, 2016 Peter Trueb 190 billion
Dec 21, 2015 Ron Watkins 130 billion
Nov 14, 2015 Ron Watkins 125 billion
Oct 12, 2015 Ethan Gallagher 120 billion
July 5, 2015 Ron Watkins 100 billion
June 13, 2015 Andreas Stiller 80 billion